You advertise your business to let people know that, "hey, I'm here and this is what I do."  Advertisement comes in many diferent forms and levels of cost.  A good sign is by far the cheapest of all the alternatives when you consider the exposure and amount of time that good signage will get, and how long it will last.  Think about the company vehicle that gets driven around, how many people see it?  What if it had your name and phone number on it?  Better yet, what if it was wrapped in a really nice graphic and became a driving billboard for your business and drew even more attention?  How much did that billboard along the freeway cost anyway?  How many people pass your storefront or the street they should have turned at to get to you?  Do you think as many people pass your ad in the phonebook as they do your place?  How long did your ad in the newspaper last?  For the same price you could of had a sign that lasted a couple of years.  Exposure is the name of the game, would it not be nice if a potential customer knew about you before they went to the phonebook and saw all your competitors?